The present invention generally relates to the use of ultrasonic pulses, or waves, to measure the properties of objects in the wave""s path, and more particularly to the use of an ultrasonic wave source to determine both the thickness of pipes and tubes and the speed of sound within the same, both from the same set of measured time of flight data and without recourse to assumed or predetermined values of properties unique to the pipe or tube being measured.
Various types of non-destructive measurement methods have been employed in the art for determining an object""s structural properties. One method, utilizing ultrasonic waves, is particularly appealing due to its accuracy, relative low cost, and safety. It is commonly used to determine object thickness, as well as to detect flaws and discontinuities. Accordingly, its use in inspecting objects with hollow internal cavities not readily amenable to visual perception or the placement of a conventional measuring device is especially valuable. Exploitation of the wavelike nature of acoustically propagated signals, when combined with knowledge of the constitutive properties of the material making up the object, leads to reasonably accurate measurement of the object. Ultrasonic testing is well-suited to the measurement of material thickness, including layered or nonhomogenous materials with acoustically disparate properties. By taking advantage of different reflective properties of the constituent materials and the speed of propagation of the sound wave therethrough, the thickness of the material can be readily calculated.
The operation of a conventional ultrasonic measurement system in the time domain is relatively straightforward. Typically, a pulse generator produces an electrical signal with certain characteristics. This signal is sent to one or more transducers, where the signal is converted to an ultrasonic wave, which is then transmitted in the direction of a target object to be measured. Typically, both the target object as well as the transducers are in an acoustically-coupled medium, such as water, to enhance the detection of the waves over a more rarified medium, such as air. Echoes reflected from the object return to the transducers, which convert the echo into a corresponding electrical signal, which is then routed to a receiver, where the signals can be counted or digitized, analyzed and stored. Analysis of the echo shows that the thickness of the object can be equated to a product of the speed of sound and the propagation time of the ultrasound wave within the object. Once any two of these three values are known, the third can be easily calculated. Nevertheless, the accuracy of some calculations based on time of flight data is subject to limitations. For example, in the conventional analysis discussed above, the speed of sound in the object being measured is assumed. Furthermore, even if the assumed speed was accurate for one temperature, it might not be for another. In addition, in the conventional analysis, the frequency-dependence of the speed of sound is not considered; the spectral content can become significant if the object being measured exhibits dispersive (i.e.: frequency-dependent) properties. These assumptions and simplifications, based on a constant, predetermined value, may introduce errors into subsequent thickness calculations. Moreover, for objects with multiple walls (such as a tube), there are limitations on placement of the ultrasonic transducers, especially where the inside diameter of the tube is small. Existing methods, while appropriate for geometrically simple structures, such as flat plates, are incapable of measuring individual wall thickness of multi-walled objects.
Current methods to measure the wall thickness of tubes and tubular-shaped objects either depend on an assumed sound velocity in the tube, or use a calibration procedure. Both of these approaches have disadvantages. In the former case, inaccuracies can result, either from improper characterization of the constitutive properties of the material in the object, or from inhomogeneities in the material itself, such as due to the presence of cladding, alloying or composite structures. Insofar as the equations used to calculate thickness depend on the speed of sound in the object being measured, any inaccuracies in that assumed quantity will produce errors in thickness calculations. In the latter case, calibration is complicated and unreliable. In addition, it often must be conducted off-line, thereby taking away from precious measurement time.
It is well-known in the art to compensate measurements for variations in the temperature of the acoustic couplant medium (typically water). However, the temperature of the water can be significantly different than the temperature of the object in the water, especially when the object is passing through the water as part of a manufacturing step, such as the extrusion of tubes and related objects. Thus, assigning a temperature value (such as the measured temperature of the water in which the object is placed) to an object being measured may not accurately reflect the true temperature within the object.
The time-domain method provides a single scalar value of the speed of sound. If the sound wave passes through a dispersive medium, its determination will depend on the frequency characteristics of the transducers used in the measurement. However, an inherent part of the time-domain method is that such frequency-dependent values are not made manifest. Thus, in certain circumstances, additional measurement accuracy can be realized by using frequency-domain analysis, which can determine the phase velocity and group velocity at different frequencies. In the present context, the term xe2x80x9cspeedxe2x80x9d, although in the strictest sense speed a scalar quantity, is used generally to represent both scalar and vector quantities in either the time-domain or the frequency-domain. Contrarily, the more specific terms xe2x80x9cphase velocityxe2x80x9d and xe2x80x9cgroup velocityxe2x80x9d are both functions of frequency, and their use is restricted to frequency-domain analyses. The frequency-dependent quantities, while usually not as big of a contributor to a thorough and accurate determination of object thickness as the speed of sound in the object, can nonetheless provide additional insight into secondary levels of measurement error, especially when conducted on dispersive materials.
Accordingly, what is needed is a method for measuring object thickness and sound velocity through the object simultaneously, such that neither assumed properties nor the use of complicated procedures is required. What is further needed is a method that eliminates the need for the associated uncertainty of thickness measurements.
These needs are met by the present invention, where a new method for determining the sound propagation velocity and wall thickness of a tubular workpiece is described. As used in the present context, a tubular workpiece need not be cylindrical in cross-section; it could be elliptical or even rectangular. It will be appreciated by those skilled in the art that the present method could also be used on elongate, non-tubular members, such as cylindrical or elliptical solid rods and related structure. Extraction of the sound velocity and tube thickness from the sensed data obviates the need for using a contact-based method for measuring the tube thickness, which can be either inconvenient or inaccurate under certain conditions. It also avoids calculation errors by utilizing actual measured speed of sound in the tubular workpiece, rather than assuming a fixed, predetermined value. It also extends the use of multiple transducers to objects with complex geometries, such as tubes, where access limitations prohibit transducer configurations for single wall measurements to be used. The method is particular useful for real-time measurement of multiple wall thickness when the sound speed of the specimen may change, such as due to a change of temperature. Unlike the prior art, where the speed of sound in the object is assumed and the measured data is corrected post facto to account for temperature variations within an acoustic couplant medium (or some other location in close proximity to the object), the data taken by the method of the present invention inherently includes temperature-related effects, as the speed of sound through the medium is a necessary concomitant of the time of flight measurements. Accordingly, temperature-related errors are minimized or abrogated entirely. In circumstances where acoustic dispersion within the tube material may be significant, this new method can provide further enhanced accuracy by taking into account the spectral content of the propagated wave through the tube walls.
According to an aspect of the present invention, a method of determining the wall thickness of a tubular workpiece and the speed of sound within the workpiece from a single set of measured time of flight data is disclosed. The method comprises the steps of configuring an acoustic couplant medium and defining a transmission path therein, and selectively disposing the tubular workpiece in the medium such that the workpiece is in the transmission path and that each interface between a surface of the workpiece and the medium along the transmission path defines an acoustic discontinuity. By selectively disposing the workpiece in the transmission path, measurements using the ultrasonic waves being transmitted with and without the presence of the workpiece can be compared. Additional steps of the method include transmitting the waves through the transmission path, receiving signals corresponding to the transmitted and reflected waves, measuring time of flight data for each of the received signals, calculating a speed of sound in the workpiece based on the measured time of flight data, and calculating a thickness of each wall of the workpiece based on the measured time of flight data and the calculated speed of sound.
Optionally, the method can include additional steps, such as compensating the speed of sound and the thickness calculations due to temperature variations in the acoustic couplant medium. The steps of the present method obviate having to rely on a predetermined value for the speed of sound. The received signals can be used in either a time-domain or frequency-domain analysis, utilizing equations specific to the respective domains to determine speed of sound (time-domain), phase and group velocities (frequency-domain) and wall thickness quantities. Preferably, the workpiece is disposed in the transmission path such that during the step of transmitting the waves, they impinge substantially orthogonal to the outer and inner surfaces of the workpiece, helping to minimize distortion errors.
According to another aspect of the invention, a system for determining the wall thickness of an object and the speed of sound within the object from a single set of measured time of flight data is disclosed. The system comprises an acoustic couplant medium configured to selectively receive a tubular workpiece therein such that the workpiece, when present, is disposed within a transmission path defined in the medium. Each interface between a surface of the workpiece and the medium defines an acoustic discontinuity. The system further includes a plurality of ultrasonic wave transducers, each configured to transmit and receive signals corresponding to time of flight data from transmitted and reflected waves, as well as signal processing apparatus operatively coupled to the transducers. The signal processing apparatus is configured to calculate the speed of sound within the workpiece based on the received signals without having to determine the temperature of the workpiece. From there, it can use the time of flight data and the calculated speed of sound to calculate the thickness of each wall of the workpiece. Optionally, the signal processing apparatus is configured to effect either a time-domain or frequency-domain analysis based on time of arrival delays of at least a portion of the ultrasonic waves.
According to yet another aspect of the invention, a method of determining the wall thickness of and speed of sound within a tubular workpiece is disclosed. Steps used in the method include configuring an acoustic couplant medium, transmitting and receiving an ultrasonic wave through the medium without the tubular workpiece present in the medium, and measuring a time of flight for the transmitted wave. Other steps include placing the workpiece in the medium, generating ultrasonic waves within the medium such that at least a portion of the waves are reflected back from at least one surface of the tubular workpiece and at least a portion of the waves are transmitted through the medium and the workpiece, detecting signals correspond to a portion of the waves reflected off the at least one surface of the tubular workpiece and a portion of the waves transmitted through the couplant medium and the tubular workpiece, measuring time of flight data for each of the detected signals, calculating a speed of sound in the workpiece based on the measured time of flight data, and calculating a thickness of the walls of the workpiece based on the measured time of flight data and the calculated speed of sound. Optionally, the method can include the additional steps of extracting the spectral content of each of the detected signals from the time of flight data, and generating output signals for a plurality of discrete frequencies within the spectral content, where the output signals are proportional to the wall thickness of at least one wall of the tubular workpiece.
According to still another aspect of the invention, a method of determining the wall thickness of and speed of sound within a hollow workpiece is disclosed. The method comprising the steps of configuring an acoustic couplant medium and defining a transmission path therein, selectively disposing the hollow workpiece in the medium such that the workpiece is in the transmission path, defining an acoustic discontinuity at each interface between a surface of the workpiece and the medium along the transmission path, transmitting a plurality of ultrasonic waves through the transmission path such that at least one of the waves being transmitted is without the presence of the workpiece in the transmission path, receiving signals corresponding to the waves reflected from at least one acoustic discontinuity and the waves that traverse the substantial entirety of the transmission path, measuring time of flight data for each of the received signals, calculating a speed of sound in the workpiece based on the measured time of flight data, and calculating a thickness of each wall of the workpiece based on the measured time of flight data and the calculated speed of sound. Optionally, the method comprises the additional steps of extracting the spectral content of each of the received signals, and generating output signals for a plurality of discrete frequencies within the spectral content, where the output signals are proportional to the thickness of the walls of the workpiece.
According to another aspect of the present invention, a method of determining the wall thickness of a tubular workpiece and the speed of sound within the tubular workpiece is disclosed. The steps of the method include configuring an acoustic couplant medium, transmitting a first ultrasonic pulse through the medium when the tubular workpiece is disposed therein such that at least a portion of the first ultrasonic pulse echoes off at least one surface of the workpiece, transmitting a second ultrasonic pulse through the medium when the workpiece is disposed therein such that at least a portion of the second ultrasonic pulse echoes off at least one surface of the workpiece, transmitting a third ultrasonic pulse through the medium when the tubular workpiece is disposed therein such that at least a portion of the third ultrasonic pulse passes diametrically through the substantial entirety of both the workpiece and the medium, transmitting a fourth ultrasonic pulse through the medium when the workpiece is not disposed therein such that at least a portion of the fourth ultrasonic pulse passes diametrically through the substantial entirety of the medium, detecting signals corresponding to the transmitted first through fourth ultrasonic pulses, measuring time of flight data for the detected signals, calculating a speed of sound in the workpiece based on the measured time of flight data, and calculating a thickness of each wall of the workpiece based on the measured time of flight data and the calculated speed of sound. Optionally, the method includes the step of wherein the first ultrasonic pulse is generated by a first transmitting device, and the second ultrasonic pulse is generated by a second transmitting device.
According to yet another aspect of the present invention, a method of determining the wall thickness of a tube and speed of sound within the tube is disclosed. The method includes the steps of selectively disposing the tube in an acoustic couplant medium; defining a transmission path in the medium, defining, an acoustic discontinuity along the transmission path at each interface between a surface of the tube and the medium, generating a plurality of ultrasonic waves at least one of which is done so without the presence of the tube in the transmission path, measuring the time of flight of the portion of the waves that reflects off the first of the acoustic discontinuities encountered along the transmission path, measuring the time of flight of the portion of the waves that reflects off the second of the acoustic discontinuities encountered along the transmission path, measuring the time of flight of the portion of the waves that reflects off the third of the acoustic discontinuities encountered along the transmission path, measuring the time of flight of the portion of the waves that reflects off the fourth of the acoustic discontinuities encountered along the transmission path, measuring the time of flight of the portion of the waves that passes through the tubular member and across the substantial entirety of the transmission path, measuring the time of flight of the portion of the waves that passes across the substantial entirety of the transmission path when the tubular member is not disposed therein, calculating a speed of sound in the tube based on the measured time of flight data, and calculating a thickness of each wall of the tube based on the calculated speed of sound and the measured time of flight data.
Other objects of the present invention will be apparent in light of the description of the invention embodied herein.